4 Volcanism Definition: the origin and movement of magmaThree categoriesExtrusive: magma is expelled onto surfaceIntrusive: magma solidifies in the shallow crust near the surface. Can be exposed after weathering.Plutonic: magma solidifies deep inside the crust.Viscosity: The ability for lava to flow. Reflective of the amount of silica present in the magma.LOW silica = LOW viscosity = nonexplosiveHIGH silica = HIGH viscosity = explosive

11 5 Types of Extrusive Volcanism 3. Caldera VolcanoFormed when the upper part of a volcano collapsesThe magma chamber below is (partially or completely) emptied after an eruptionThe emptied magma chamber can no longer support the weight of the overlying rockThe overlying rock collapses into itself, forming a circular basinVolcanic activity can continue if there is still magma in the chamber belowExample: Crater Lake, OR

15 5 Types of Extrusive Volcanism 5. Lava Dome“Plug Dome”High-silica lava exits from a vent. Due to its high viscosity (doesn’t flow very fast or far), it builds up into a dome.Effectively “plugs” the main vent. Can still emit ash, gases, and even lava.Example: Lava Dome in Mt St Helen’s crater.

19 Dangers of Volcanic EruptionsBurning Hot LavaEarthquakes, Landslides and TsunamisThese are often related – an earthquake might jar some land loose, resulting in a landslide. The earthquake may set off the tsunami. Also, if the landslide falls into the ocean, can trigger a tsunami.Mudslides (“Lahars”)Ash FalloutPyroclastic Flow (“nuée ardente”)

23 Dangers of Volcanic Eruptions Pyroclastic flow example: Mt St Helens 1980 eruptionHot gases, ash, rock that become a “fluid cloud” that move down the side of the volcano. Can travel great distances.“nuée ardente”: French for “glowing cloud”